The present invention relates generally to the data processing field, and more particularly, relates to a method and apparatus for enhanced performance in the presence of stationary position error signal (PES) outliers in a direct access storage device (DASD).
Direct access storage devices (DASDs) often incorporating stacked, commonly rotated rigid magnetic disks, are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a radial path toward and away from the drive axis write data to the disks and read data from the disk surfaces. Typically servo information is provided on one or more disk surfaces for reading by the transducer heads for accurately and reliably positioning transducer heads on the disk surfaces at a specific location to read and write data.
Typically the actuator servo information is demodulated into a position error signal (PES) and used for radial positioning of the recording head. The servo information, for example, is written in equally spaced sectors around the tracks. The sectors are lined-up almost perpendicular to the concentric circular tracks. Around the track, the servo sectors are interlaced with much larger data sectors. Occasionally the computed value of the position error signal (PES) derived from a particular servo sector has a value that lies far outside the expected value. A PES having this property is referred to a PES outlier. The PES outliers can be separated into transient and stationary outliers.
The presence of stationary PES outliers is often the cause of soft or sometimes hard data errors due to poor track misregistration (TMR). There are also many other problems caused by stationary PES outliers. First the stationary PES outliers are repeat offenders causing a performance degradation due to extra time spent on error recovery procedures (ERP). The stationary PES outliers cause problems for seek and settling actuator operations. Also the presence of a PES outlier in the PES data used for the generation of the feedforward sequence, required by the actuator servo system, will result in an undesirable phase shift in the feedforward sequence. However, no systematic method and apparatus exists to identify stationary PES outliers.
A need exists for a mechanism to facilitate enhanced performance in the presence of stationary position error signal (PES) outliers.
A principal object of the present invention is to provide a method and apparatus for enhanced DASD performance in the presence of stationary position error signal (PES) outliers. Other important objects of the present invention are to provide such method and apparatus for enhanced performance in the presence of stationary position error signal (PES) outliers substantially without negative effect and that overcome many of the disadvantages of prior art arrangements.
In brief, a method and apparatus are provided for enhanced performance in the presence of stationary position error signal (PES) outliers in a direct access storage device (DASD). A plurality of PES samples are obtained from a plurality of servo sectors around a track. Utilizing the PES samples, a normalized outlier value is computed for each of the plurality of servo sectors around the track. The computed normalized outlier values are sequentially stored and compared with a set threshold value. A stationary PES outlier is identified responsive to each stored outlier value being greater than the set threshold value. The effect of each stationary PES outlier is eliminated or reduced during predefined operations of the DASD.
In accordance with features of the invention, during a recalibration mode of the DASD, a different track for feedforward generation is selected to avoid an identified stationary PES outlier. When more than one stationary PES outlier is identified from the servo sectors around a track, then data from the track is reassigned to an alternate track and the flawed track is marked to not be used for future data storage.